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Author Topic: Ideas/ thoughts for ground mounted masts.  (Read 896 times)
AB4D
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« on: December 27, 2007, 07:08:18 PM »

Hello all,

I have a fenced in two acre field without trees that I want to install some type of ground mounted, self supporting, fold over masts (four), to support a wire loop for 160 meters, at about 50+/- feet.  I was thinking of using pressure treated wood. Whereas, for the base, I would sink two 4X6X10 posts spaced eight inches apart, four feet deep into the ground, run a large grade 8 bolt through both posts near the top to act as a pivot point for the mast, and use another bolt near the bottom to lock the mast vertical, the mast would start out as 2X6's, three boards thick for 16 feet, taper to 4x4's, three boards thick for 16 feet, three 2x3's, three boards thick for 12 feet, and finally use a single replacable 1x1 for 10 feet or so to support a rope/pulley.  All boards except the 1X1 would be glued and attached with stainless screws, all joints would use S/S plates and bolts on both sides of each joint, and there would be at least a four foot overlap of the center board between joints.

Does this idea seems flawed?  Thanks for any input and thoughts.

73 Jim        
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KA5N
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« Reply #1 on: December 27, 2007, 07:48:42 PM »

Have you perchance, ever weighed pressure treated lumber?  You are going to have a lever with a 10:1 ratio disadvantage.  It will be hell for stout but near impossible to raise.  I have seen masts of much smaller/lighter lumber (doubled 2X4's) forty feet high which survived quite well for several years.  Of course they had to be guyed, but they were very secure and relatively easy to raise and lower (hinged near the ground) and much cheaper than what you have planned.  Power poles are another possibility.  They don't have to be guyed and will last for years.  A crew can auger the holes and set the poles in short order.  I am sure there are other possibilities such as the lamp posts you see in parking lots.
Good luck
Allen
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W3LK
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« Reply #2 on: December 27, 2007, 07:50:56 PM »

I don't know about the rest of the mast sections, but a 10' 1x1 will be VERY limber (even out of hardwood) - too limber to support any sideways pull without a corresponding guy on the opposite side.

Even a 10' 1.5" round pole will flex quite a bit when you put a strain on the end.

73,

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K9KJM
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« Reply #3 on: December 27, 2007, 11:49:46 PM »

Your plan should work OK **only** if you also back guy each post, As already mentioned.

I would get some used "Pole line hardware" (Galvanized steel 5/8" diameter) Bolts for this project. From a cable TV construction crew, Telephone line crew, Or power company line crew. These type of 5/8" pole bolts, washers, and nuts are tossed in the dumpsters every day. They are designed for outdoor use in wood. I would not go out and buy "grade 8" bolts........  Save money AND have a better result.
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KB9CRY
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« Reply #4 on: December 28, 2007, 06:07:50 AM »

 to support a wire loop for 160 meters, at about 50+/- feet.


With that much open space available, I'd scrap the low loop plan and install a 4 square for 160.  Big big difference in performance.
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K4SAV
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« Reply #5 on: December 28, 2007, 11:58:25 AM »

Based on the measured weight of pressure treated 4x4s that I have, the weight of pressured treated lumber is
W = 0.223 (A)(L),  where A = cross sectional area in sq inches, and L = length in feet
Because pressure treated wood varies a lot in weight, this could easily be +/- 30%.

Using an average weight value, the weight of the sections would be:
The 3x2x6's would weigh 128 lbs
The 3x4x4's would weigh 171 lbs
The 3x2x3/s would weigh 48 lbs
The 1x1 would weigh 2.2 lbs

From pivot point to the attachment point of the winch cable would be about 5 ft. That would require about 5400 lbs at a right angle to the mast to raise it, or 7660 lbs at a 45 degree angle which the winch would have to supply and the cables and attachments points would have to sustain.

These same forces also exist on the bottom supports and against the dirt in a sideways direction.

At the bottom, this is equivalent to suspending a Ford Expedition 5 ft out on the end of a 6x6 (also supported by something placed 5 ft away, 4 ft down in dirt).

A few comments:
The 1x1 will not be strong enough to support anything.  This will have to be either beefed up or the top section eliminated and make the mast  44 ft.  Beefing it up will increase the raising force significantly.  

The second section of this mast is thicker and heavier than the bottom section, which doesn't sound correct.

I doubt that these sections are strong enough to withstand the forces involved in the raising process (although I do not have any measured data).  Even if they were strong enough, a large concrete base will be required to keep the thing from overturning.

You also need to do calculations for wind loading.  This is a broad structure and will incur lots of wind loading.  I would expect the wind loading to be much greater than the forces involved in raising it.

Note: These numbers are a little in error.  I used the dimensions given as actual.  For example, a 2x6 isn't really 2 by 6 inches, it is less, so the calculations need to be redone for little more accuracy, but I don't think it will change any conclusions.

Jerry, K4SAV
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AA4PB
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« Reply #6 on: December 28, 2007, 12:31:58 PM »

My experience with pressure treated lumber in building decks and such is that it tends to twist and warp as it dries out unless its firmly secured in place. I wouldn't expect the proposed supports to be very straight after a year or so.
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N3BIF
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« Reply #7 on: December 28, 2007, 12:56:05 PM »

 If your ok looking at what you have proposed in the corners of your yard, then you wouldn't mind four utility poles out there. AS touched on before poles would be a much stronger and a crew could drop them in much faster and easier then you could ever hope to build your masts.
     Just add a pulley and lead at the top of each one before they are planted, plus you would be able to climb these with hooks if ever the need arises.
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WW5AA
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« Reply #8 on: December 28, 2007, 01:35:38 PM »

I'm with Philip, kb9cry. A four square would be the way to go. It would be less trouble to install, last longer and more effective than a low loop.

73, de Lindy
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K4SAV
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« Reply #9 on: December 28, 2007, 02:45:52 PM »

In reference to some of the suggestions and comments:

.."My experience with pressure treated lumber in building decks and such is that it tends to twist and warp as it dries out unless its firmly secured in place. I wouldn't expect the proposed supports to be very straight after a year or so."...
 
True.  I have a birdhouse supported by a 10 ft mast mounted to a 10 ft 4x4.  After a year and a half, it is leaning at about 15 degrees due to 4x4 warpage and twist.

----------------
Power poles are certainly capabile of handling this, but they are very expensive, more than a self supporting tower (unless you have a friend at the power company).

Two acres is not enough to put up a four square for 160 (you need about 3.5 acres), but it is enough for an 80 meter four square.

The cheapest way to get supports is to use Rohn H50 masts (they are 44.5 ft).  You have to use guy wires, but compared to the cost of other solutions, it may be a good compromise.

Jerry, K4SAV
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WB2WIK
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« Reply #10 on: December 28, 2007, 03:50:52 PM »

Four 60' aluminum self-supporting towers are lightweight, very strong, inexpensive, and don't require any guys.

I'd do that.

In fact, I've done that!

WB2WIK/6
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